Since the discovery of carbon nanotubes, several methods have been reported for the synthesis of multi-walled, double-walled and single-walled carbon nanotubes. Methods for synthesis of carbon nanotubes include arc-evaporation of graphite, laser ablation, chemical vapor deposition (CVD) and vapor phase decomposition or disproportionation of carbon-containing molecules. Among the various types of carbon nanotubes, single-walled carbon nanotubes (SWNTs) are of special interest because of their unique properties and potential applications. Single-walled carbon nanotubes exhibit electronic, thermal, and mechanical properties with several possible applications. SWNTs may be semiconducting, semimetallic, or metallic depending on the geometrical structure. Applications of SWNTs can generally be divided into semiconducting applications and metallic applications. Unfortunately, most, if not all, formation methods provide a mixture of both metallic and semiconducting SWNTs. As such, great interest is currently being shown in the development of methods that produce only one or the other type of SWNT as well as in the development of methods to separate the two species following formation.
The spatial orientation of the carbon nanotube is represented by a pair of indices (n,m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of the graphene sheet. The values of n and m determine the chirality, or “twist” of the nanotube. The chirality in turn affects the conductance of the nanotube, its density, its lattice structure, and other properties. Nanotubes having chiral numbers n=m are metallic and quasi-metallic if n−m is divisible by 3, while all the other nanotubes are generally semiconducting. In conventional synthetic processes for SWNTs, such as the carbon arc discharge method, metallic nanotubes constitute about one-third of SWNTs, the remaining being semiconducting nanotubes. The use of SWNTs in electronics, sensor technology and other areas depends on whether the nanotubes used are metallic or semiconducting. Metallic SWNTs may function as conductive additives and as leads in nanoscale circuits, while the semiconducting ones may be used to design field effect transistors. SWNTs mixtures of metallic and semiconducting nanotubes limit their applicability. Even in those processes such as the dual laser pulse method, that preferentially form one type of SWNT over the other, the product will generally still contain some portion of the other type, and the presence of the undesired type in the product sample can frustrate if not completely destroy the desired application of the product SWNTs.
Accordingly, in the last few years, several methods have been devised to separate semiconducting and metallic nanotubes. Methods to separate metallic and semiconducting SWNTs include dielectrophoresis, density gradient centrifugation, ultracentrifugation, and selective destruction of one type of nanotube by irradiation or by chemical means, selective interaction with molecules and covalent or non-covalent functionalization.
Other methods include preferential charge-transfer interaction of bromine with the metallic species over the semiconducting species in surfactant-stabilized SWNTs, followed by centrifugation, has been used to separate semiconducting from metallic SWNTs. Additionally, derivatized porphyrins have been used which selectively interact with semiconducting SWNTs through non-covalent interaction. Such interaction may be employed to dissolve the semiconducting species in organic solvent, leaving the metallic species as residue. Photochemical osmylation has been employed to selectively react metallic SWNTs with osmium tetroxide. Subsequent self-aggregation results in the separation of the metallic and semiconducting SWNTs. However, these methods do not yield a high purity and repeat performance of the procedure is required for effective separation.
Other techniques attempt to effect the separation of semiconducting and metallic SWNTs by employing fluorous chemistry, in which the diazonium salt of 4-heptadecafluorooctylaniline reacts preferentially with metallic SWNTs present in the mixture of nanotubes. However, this protocol is very complicated.
Furthermore, these approaches do not always allow bulk scale separation with high selectivity and require cumbersome ultracentrifugation. It is, therefore, highly desirable to find a simple and scalable strategy for the separation of SWNT mixtures.